Electrical submersible pumps (ESP) are commonly used in hydrocarbon producing wells. A typical ESP includes an electrical motor having a rotating drive shaft that drives the pump. The pump may be a centrifugal pump or other types, such as a progressive cavity pump or even a reciprocating pump. The motor is filled with a dielectric motor lubricant, and a pressure equalizer reduces a pressure differential between the motor lubricant and the well fluid on the exterior. The pressure equalizer is usually located between the motor and the pump, but it could alternately be located below the motor.
A mechanical face seal is normally employed to seal the rotating drive shaft from the entry of well fluid into the motor. A mechanical face seal has a rigid rotating member that is urged by a spring against a rigid stationary base. Some leakage past the interface between rotating member and the stationary base is required to lubricate the seal and reduce heat. This arrangement usually results in some leakage of well fluid into the lubricant within the motor. This leakage of well fluid can be greatly exacerbated by vibration from the pump, abrasives present in the well fluid, and incompatibility of the sliding seal material with the well fluid. The presence of well fluid in the motor lubricant is damaging to the motor.